This invention relates to fuel cells and, more particularly, to fuel cells incorporating a solid polymer electrolyte membrane to conduct protons between the electrodes of the fuel cell and including wicking strands to hydrate the membrane.
Work is commonly derived from fuel by a combustion process which uses the pressure of expanding gases to turn a turbine or move a reciprocating piston and, ultimately, provide torque to a driveshaft. This torque is then usually used for propulsion or to generate electrical power. In the latter case, the electrical power is oftimes reconverted into mechanical work.
The by-products of the combustion process are waste gases which contaminate the atmosphere or, if pollution is to be avoided or at least reduced, reacted with catalysts to produce benign compounds. The foregoing process is usually expensive and typically calls for operations and equipment that require extensive monitoring and maintenance to ensure that the emission of pollutants is kept below a prescribed maximum. Furthermore, there are energy losses inherent in the use of expanding gases to drive a turbine or piston engine due to the inefficiency of the combustion process and friction of moving parts.
One approach which avoids the foregoing disadvantages inherent to generating work by burning a fuel is the fuel cell, which produces electrical power directly from a chemical reaction which oxidizes a fuel with the aid of a catalyst. No intermediate steps, such as combustion, are needed, nor is the machinery to generate electrical power from the torque of a driveshaft. The chemical energy of the fuel is utilized much more efficiently. Since polluting waste gases are not emitted, the attendant processes and equipment required to neutralize these harmful by-products are unnecessary.
The simplest fuel cell consists of two electrodes separated by an electrolyte. The electrodes are electrically connected through an external circuit, with a resistive load lying in between them. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly, or xe2x80x9cMEA,xe2x80x9d consisting of a solid polymer electrolyte membrane, or xe2x80x9cPEM,xe2x80x9d also known as a proton exchange membrane, disposed between the two electrodes. The electrodes are formed from porous, electrically conductive sheet material, typically carbon fiber paper or cloth, that allows gas diffusion. The PEM readily permits the movement of protons between the electrodes, but is relatively impermeable to gas. It is also a poor electronic conductor, and thereby prevents internal shorting of the cell.
A fuel gas is supplied to one electrode, the anode, where it is oxidized to produce protons and free electrons. The production of free electrons creates an electrical potential, or voltage, at the anode. The protons migrate through the PEM to the other electrode, the positively charged cathode. A reducing agent is supplied to the cathode, where it reacts with the protons that have passed through the PEM and the free electrons that have flowed through the external circuit to form a reactant product. The MEA includes a catalyst, typically platinum-based, at each interface between the PEM and the respective electrodes to induce the desired electrochemical reaction.
In one common embodiment of the fuel cell, hydrogen gas is the fuel and oxygen is the oxidizing agent. The hydrogen is oxidized at the anode to form H+ ions, or protons, and electrons, in accordance with the chemical equation:
H2=2H++2exe2x88x92
The H+ ions traverse the PEM to the cathode, where they are reduced by oxygen and the free electrons from the external circuit, to form water. The foregoing reaction is expressed by the chemical equation:
xc2xdO2+2H++2exe2x88x92=H2O
One class of fuel cells uses a solid PEM formed from an ion exchange polymer such as polyperfluorosulfonic acid, e.g., a Naflon(copyright) membrane produced by E. I. DuPont de Nemours. Ion transport is along pathways of ionic networks established by the anionic (sulfonic acid anion) groups that exist within the polymer. Water is required around the ionic sites in the polymer to form conductive pathways for ionic transport.
The ionic conductivity of such PEMs is thus a function of the water content in the polymer. More particularly, the conductivity will decrease as the water content drops below a minimum threshold level. As the conductivity drops, the efficiency of the fuel cell decreases until, if the polymer becomes excessively dry, the fuel cell becomes non-conductive.
There are several factors causing the removal of water from the anode surface of the PEM. Heat generated in the oxidation reaction, as well as by transport of the free electrons, i. e., IR type losses, causes evaporation. Water is also lost through electroosmotic transport by hydrogen water compounds, e.g., xe2x80x9chydronium ions,xe2x80x9d (H3O)+. This is a process in which water molecules are xe2x80x9cdraggedxe2x80x9d through the PEM by hydrogen protons migrating from the anode to the cathode. Each H+ ion is believed to transport one or two water molecules along with it through the mechanism of electroosmotic xe2x80x9cdrag.xe2x80x9d
Dehydration of the membrane is a problem endemic to PEM fuel cells. Abundant water collects on the cathode from being created by the reduction of the H+ ions and from being transported through the PEM by electoosmosis, and some of this water will automatically migrate back through the PEM to the anode by virtue of the mechanism of diffusion. However, the rate of water migrating back to the anode by means of diffusion is not always sufficient to prevent excessive PEM drying under high current operating conditions, and thus diffusion alone cannot be relied upon to prevent drying under the range of operating conditions that a fuel cell might be expected to encounter.
One approach to maintaining adequate hydration of the PEM is to use a humidifier external to the fuel cell structure to introduce water as steam or a fine mist into the stream of hydrogen gas fuel flowing into the anode. Another method is to bubble the fuel gas through water kept at a temperature higher than the temperature at which the fuel cell is operated.
There are, however, limits on the effectiveness of humidification as a viable solution occasioned primarily by constraints inherent to the mechanism of condensation. More particularly, water condenses in the anode in an amount corresponding to the difference between the saturation vapor pressure of the humidified fuel gas at a humidification temperature and the saturation vapor pressure at a cell operation temperature. The difference between the humidification temperature and the cell operation temperature is typically too small to provide for condensation sufficient to avoid excessive dehydration of the PEM.
One solution has been to increase the temperature differential by increasing the humidification temperature. However, the increased humidification temperature causes an increase in the partial pressure of the water vapor which is greater than the attendant increase in the partial pressure of the fuel gas. This unequal increase in partial pressures causes a decrease in the quantity of fuel gas per unit of volume in the humidified gas mixture entering the fuel cell which, in turn, adversely affects the performance of the fuel cell.
Moreover, even with the gaseous mixture being saturated with water in an amount sufficient to prevent dehydration of the PEM, and assuming arguendo that the quantity of condensed water is similarly adequate, the application of the condensed water is not uniform over the surface of the anode. Rather, most of the water condenses on the part of the anode nearest incoming stream, leaving the more distant portions of the PEM subject to drying out.
The quantity of moisture carried by a saturated gas, and thus the amount of condensed water, can be increased by increasing the flow rate of the saturated gas, but this requires a recirculating gas system including recirculation pumps and some means of filtering impurities which tend to accumulate in the unused gas recirculating through the system. A significant drawback to humidifying the fuel gas, and to a recirculation system in particular, is the necessity for pumps, valves, heaters and other equipment which add to the overall cost of the fuel cell in addition to increasing its weight and adversely affecting its reliability.
An alternative to humidifying the fuel gas is to direct a stream of water across the anode. However, as there is no feedback as to the rate the liquid water is being absorbed into the PEM, this approach typically delivers much more liquid water to the anode surface than the quantity being absorbed. This excess of liquid water restricts access of the fuel gas to reaction sites on the anode and consequently has an adverse effect on the performance of the fuel cell.
Wicks also have been used to conduct water from a reservoir to the surface of the anode and PEM. More particularly, U.S. Pat. No. 5,534,363 discloses a wicking structure comprised of hollow tubing having porous walls. The tubing is completely covered by a porous fabric or foam having numerous tiny fabric or foam fingers emanating from the surface of the cover.
The cover adheres to the surface of the fuel cell""s anode sheet, and is formed from a material capable of bonding to the porous tube and the anode surface. Thus, the cover material must be selected in view of the material used to form the anode sheet. As noted in column 6, lines 38-46, selecting the proper anode wick materials may require dismantling and analysis of the anode because its physical and chemical characteristics may be a trade secret.
In column 7, lines 13-28, an alternative embodiment discloses a wick comprised of hair shaped tubing formed into the shape of a tree trunk with branches. This tubing is perforated with small holes, and may or may not require porous fabric or foam covers depending upon the wetting characteristics of the anode material and the PEM""s hydration requirements. As no alternative mechanism is disclosed to affix this alternative wick structure to the surface of the anode, it is implied that the tubing also must be composed of a material that will bond with the anode sheet.
Another alternative anode wick design replaces the porous fabric or foam fingers with cloth knotted from hydrophilic (i.e., water absorbent) thread and hydrophobic (i.e., water repellent) thread. The knit cloth contains repetitive square hydrophobic regions which substantially exclude all liquid water and allow the passage of hydrogen gas to the anode. Surrounding each hydrophobic region are zones of hydrophilic stripes which exclude hydrogen gas and allow liquid water to be transported via wicking action from the hollow tubing to the anode surface.
As noted, the wicks of the foregoing patent use hollow tubing which lies on the surface of the anode and is affixed thereto by adhesion. It requires an analysis of the anode material because the tubing or tubing cover must be formed from material which will bond with the anode. To use such a wick, various fuel cells must each be dismantled and have their anodes analyzed to ensure proper bonding. A wick must be specially manufactured to be used with each fuel cell having an anode composed of a material having unique bonding characteristics.
U.S. Pat. No. 5,322,744 also discloses a wick to supply water to an anode and, ultimately, to a PEM. More specifically, in column 10, lines 1-4, the patent discloses a wick xe2x80x9c. . . made of fine threads of a fibrous material preferably selected from various natural fiber, synthetic fiber or metallic fiber, the fine threads having been twisted together.xe2x80x9d The twisted threads form a bundle. Any object on the surface of the anode or PEM, such as bundled threads forming a wick, decreases the surface area otherwise available for the transport of H+ ions, and proportionally reduces the current density of the fuel cell. Thus the thicker the bundle, which is obviously thicker than the individual threads, the more the current density is reduced and fuel cell performance compromised.
The water is conducted between the individual fibers forming the bundle along a spiraling, tortuous path, in contradistinction to a linear path. The aforementioned nonlinear flow path requires the water molecules to travel a longer path to reach any part of the surface of the anode, particularly with respect to the area of the anode and adjacent PEM lying the furthest distance from the water source for the wicking bundle. This requires more work from the capillary force driving the water molecules and more time for the water to traverse the distance from the water source to the PEM.
U.S. Pat. No. 5,358,799 discloses the use of a capillary wick to conduct water to the anode and from the cathode of a fuel cell. At column 5, lines 59-60, it states that the capillary wicks are comprised of xe2x80x9cporous graphite or other suitable materials.xe2x80x9d
Several references of the prior art disclose a fiber or strand having three hollow interior regions extending axially for the length of the strand, formed by three T-shaped partitions intersecting at the strand""s core. In particular, FIG. 3 of U.S. Pat. No. 5,759,394 illustrates the foregoing fiber. The fiber entraps a solid absorbent within the longitudinal regions. The absorbent absorbs undesirable molecules from a passing air stream. Wicking of liquid is not disclosed or suggested.
In U.S. Pat. No. 5,891,221, the aforementioned fiber configuration is shown in FIG. 3 in conjunction with carrying and retaining a liquid having an affinity for undesired odor causing gas phase molecules. The fiber uses capillary action to draw the selected liquid with which it comes into contact through its interior regions. The liquid removes the undesired gas phase molecules from air passing around and through a bundle of the fibers.
FIG. 3 of U.S. Pat. No. 5,704,966 also shows a wicking fiber having the aforementioned trilobed configuration. A bundle of the fibers disclosed therein is used to filter gaseous contaminants from an air stream. Each fiber contains a liquid which captures the gaseous contaminants. The fiber carries the liquid containing the captured contaminants to another air stream which strips them from the fiber and carries them away.
A fiber having the aforementioned trilobed configuration is shown in FIG. 1 of U.S. Pat. No. 5,057,368. At column 5, lines 20-21, liquid wicking is noted as one of its applications. FIG. 5 of U.S. Pat. No. 5,713,971 also shows a trilobed fiber having the aforementioned configuration. At column 4, lines 58-62, this reference discloses using capillary force to wick a liquid up the interior of the fiber. The liquid is to have an affinity for the undesirable material to be removed from an air stream.
The latter four references each discloses a trilobed fiber having a fiber or strand having three hollow interior regions extending axially for the length of the strand. Each of these references notes that the fiber disclosed therein can wick liquid. However, none of them disclose or suggest the use of such a fiber to solve the long standing problem of adequately hydrating the PEM of a gas fuel cell.
As may be seen from the foregoing, there presently exists a need in the art for a hydration apparatus which keeps the PEM of a fuel cell sufficiently hydrated while overcoming the shortcomings, disadvantages and limitations of the prior art. The present invention fulfills this need in the art.
Briefly, in a fuel cell a PEM is located between two layers composed of a catalyst material such that a sandwich-like assembly is formed. The fuel cell further includes two electrodes, each comprised of a thin sheet of porous material that is permeable to liquid and gas. The two electrodes are situated on either side of the sandwich-like assembly such that one surface of each electrode abuts a catalyst layer.
The remaining surface of each electrode respectively abuts a nonporous bipolar plate. The bipolar plate has grooves for gas flow, and serves as a manifold to distribute fuel gas across the abutting electrode. The two bipolar plates are conductive, and are electrically connected by an external circuit.
Wicking strands are located in between each catalyst layer and the adjacent PEM. Each strand is composed of a trilobed fiber. The strands are arranged in a repetitive pattern such that they do not cross over or overlap each other.
Hydrogen fuel gas flows through the grooves in the anode bipolar plate, diffuses through the anode electrode, and reacts with the catalyst to produce free electrons and H+ ions. The electrons flow to the cathode electrode by means of the external circuit, and the H+ ions migrate through the PEM to the cathode electrode. The wicking strands abutting the PEM surface facing the anode electrode communicate liquid water from a reservoir to the foregoing PEM surface to maintain adequate hydration of the PEM.
Oxygen gas flows through the grooves of the cathode bipolar plate and reacts with the H+ ions and free electrons to form liquid water. The wicking strands abutting the PEM surface facing the cathode electrode communicate liquid water from the surface of the PEM to an exhaust reservoir.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.